Bump webpack-dev-server from 2.11.1 to 3.1.11 (#3104)

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 2.11.1 to 3.1.11.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack/webpack-dev-server/compare/v2.11.1...v3.1.11)

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -77,7 +77,7 @@
     "to-snake-case": "^1.0.0",
     "uglifyjs-webpack-plugin": "^1.1.8",
     "webpack": "^3.11.0",
-    "webpack-dev-server": "^2.11.1"
+    "webpack-dev-server": "^3.1.11"
   },
   "peerDependencies": {
     "slate": "*",